As shown in FIG. 1, in a DOCSIS Passive Optical Network (DPON) system 100, DOCSIS downstream data from a headend 102 (of, e.g., a cable television system) is sent to the end user premises 101 via an analog laser 110 (operating at, e.g. 1550 nm), which carries Quadrature Amplitude Modulation (QAM) modulated signals that could be either video or data. Upstream data from each user premises' optical network terminal (ONT) 108 are sent back at precise allocated time slots, assigned by a cable modem termination system (CMTS) 130 at the headend 102, as in a Time Division Multiplex (TDM) system. The upstream transmitter laser 125 in each ONT 108 is turned on during its assigned transmission time, and off otherwise. All upstream transmitter lasers 125 could have the same wavelength (e.g. 1310 nm) or different wavelengths (in which case a dense wavelength division muliplexer (DWDM) would be employed in place of the simple 1-N optical splitter 120).
As further shown, upstream data passes through an optical fiber 150 that is connected to an optical multiplexer 132, which allows upstream and downstream data to coexist on the single optical fiber 150. Upstream data is passed to an optical receiver 134 and converted to electrical signals and passed to the CMTS 130 as a radio frequency (RF) signal (typically in the 5 MHz-42 (or 85) MHz band). The upstream data is originally generated at the ONT 108 in a cable modem/data processor 128 and an optical diplexer 122 operates such that the ONT 108 can communicate upstream and downstream using the same optical fiber 150.
On the downstream side, the CMTS 130 produces a downstream RF signal (typically in the 50 MHz-1002 MHz band) and passes that signal to laser driver 110 (which, as noted, may be a 1550 nm laser). The resulting optical signal makes its way to ONT 108 via optical multiplexer 132, optical fiber 150 and 1-N optical combiner 120, and is split from any upstream data at the ONT's optical diplexer 122, whereupon the downstream data is passed to receiver 127, and then on to cable modem/data processor 128.
Upstream data could be sent via analog lasers or digital lasers. In the case of analog lasers, reverse QAM modulated signal output from cable modem 128 may simply be used to drive analog lasers directly. The converted optical signal is sent back to the headend 102, where the optical signal is converted back to electrical RF signal, then fed to the CMTS 130, as explained. Though simple, the usage of analog lasers requires lasers with high gain, deep reach, low noise, and hence, are quite expensive, and therefore not that attractive in a DPON network.
Digital laser use in the upstream path offers certain advantages over analog lasers. The theoretical signal-to-noise ratio of a digitized signal is:6.02N+1.76dB+10log10{(fs/(2*Bw)},  (Equation 1)where N is the number of bits used to digitize the analog signal, and fs is the Nyquist bandwidth. As implied in the above equation, the signal-to-noise ratio could be increased by raising the number of digitized bits N, effectively increasing the signal strength. The last term of the equation also shows that a 3 dB processing gain could be obtained each time fs is doubled. The abundant availability of digital lasers and analog-to-digital (A/D) converters offer a more robust and cost effective solution for upstream data transport in DPON as compared to using analog lasers. FIG. 2 shows a typical DPON system that uses digital lasers in the upstream portion. This system is similar to the one shown in FIG. 1, except that Serializer/Deserializer (SERDES) devices 202, 204 are used at each ONT 108 and at the headend to transmit and receive digitized serial signals.
As mentioned, and as shown in FIG. 3, respective upstream data bursts 302, 304, 306, 308 from ONTs 108 are sent back in a time-division-multiplex scheme. In the case of a DOCSIS compliant system, each data burst 302, 304, 206, 308 follows DOCSIS standards. And, in the case of a digital DPON system, each burst consists of serial bits in GHz data rates.
Improvements to this architecture and methodology are desirable.